Control systems for battery operated motors including an SCR chopper circuit connected in series with the motor across the battery are well-known in the art. Such SCR chopper circuits include a capacitor and first silicon controlled rectifier connected in series with each other across a second silicon controlled rectifier and function as a pulse generator. Such control systems include means for adjusting the pulse frequency of the SCR chopper and thereby the mean power applied to the motor during operation. Such control systems also include means, of which the capacitor is a part, for turning the SCR chopper on and off depending on whether power to the motor is desired or not. The SCR chopper includes means operative upon turn-on thereof to stop the inductive effects on the battery from reducing the charge on the capacitor which would prevent the subsequent turn-off of the SCR chopper.
However, when systems including two or more SCR chopper actuated electrical devices, such as direct current motors, connected in parallel across a common battery are operated under the most demanding conditions, one of the SCR choppers will sometimes fail to turn off the motor actuated thereby when desired. For example, battery powered lift trucks including a drive motor and a lift or "pump" motor each actuated by a different SCR chopper are widely used and under ordinary circumstances operate without fault. However, applicant has found that under high stress circumstances, the operator may suddenly lose control of one of the motors. Thus, if the operator is trying to inch slowly up a steep ramp and, at the same time, intermittently raise the lift, there have been instances of the lift motor suddenly turning on at full speed and not responding to the turn-off control. The safety considerations raised by such an incident are very substantial.
Applicant has discovered that such incidents result from the coincidence of signals turning on one SCR chopper to actuate one motor and turning off the other SCR chopper. As is well-known in the art, the turning on of an SCR chopper results in a dip in the battery voltage due to inductive effects in the system and applicant has discovered that if a second SCR chopper powered by the same battery is turned off either during such dip or slightly before such dip occurs, there will be insufficient charge on the capacitor of the second SCR chopper to turn it off on its next cycle of operation, in spite of the means included in such SCR chopper to stop the inductive effect when operated by itself.